1. Field of the Invention
The invention relates generally to the field of exploration and production of hydrocarbons from wellbores. More specifically, the present invention relates to a method and apparatus to position and secure production and exploration tools within a wellbore.
2. Description of Related Art
Perforating guns are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore, and the casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.
Included with the perforating guns are shaped charges that typically include a housing, a liner, and a quantity of high explosive inserted between the liner and the housing. When the high explosive is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge at very high velocity in a pattern called a “jet”. The jet penetrates the casing, the cement and a quantity of the formation.
Generally the wellbore pressure is different from the pressure within the geological formation that is to be perforated, thus upon perforation pressure equalization occurs between the formation and the wellbore which in turn produces either flow into the wellbore from the formation, or into the formation from the wellbore. When the wellbore pressure is greater than the formation pressure this is known as an overbalanced situation, whereas when the formation pressure exceeds the wellbore pressure is known as an underbalanced situation.
Many times when the perforating guns are detonated, the forces applied to the perforating guns are not balanced and can produce a resultant force that thrusts the perforating gun suddenly upward or downward upon detonation. This can be exacerbated in an overbalanced or underbalanced condition. Attempts have been made in the past to prevent perforating guns from being jolted within the wellbore; such as by securing or anchoring the guns within the wellbore just prior to firing the perforating guns. These attempts include adding anchoring devices to perforating guns where the anchoring devices are actuated mechanically, electrical, or hydraulically. However all of the previously developed devices suffer from one or more of the following disadvantages. For example, electrical or hydraulically activated anchors are susceptible to problems with reliability, and none of the above noted devices is capable of resetting its anchor should the perforating gun receive an impulse from an unexpected direction. Examples of these devices can be found in the following patents: U.S. Pat. Nos. 6,314,043, 5,971,070, 4,554,975, and 4,284,137.
Some anchors are designed to prevent movement in a single direction, these would be used in a highly overbalanced or highly underbalanced case. In overbalanced wellbores it is expected that subsequent to perforating the wellbore, the higher pressure wellbore fluids would quickly migrate into the surrounding formation, that in turn would pull the perforating gun downward and stress the wireline attached to the perforating gun. Thus in overbalanced cases, when using an anchor that prevents movement in only one direction, the anchor would be positioned to prevent downward movement of the perforating gun. However if the forces from the individual jets fired from the perforating gun produce a resultant upward force onto the perforating gun, the anchor can become unseated. If the anchor becomes unseated, the perforating gun will be thrust upward for some distance and then begin to fall, unsupported, within the wellbore. During the ascent of the perforating gun, slack will accumulate in the wireline above the perforating gun. After the perforating gun begins to fall, it will begin to drop within the wellbore until no slack remains in the wireline. Depending on how much speed the perforating gun has attained, it can break the wireline when the slack in the wireline runs out and the perforating gun pulls the wireline taut.
Therefore, there exists a need for a device that reliably anchors a perforating gun within a wellbore during perforations, and is capable of resetting the anchor without the threat of damaging the wireline.